Elastic boundary wave device

ABSTRACT

An elastic boundary wave device includes a first medium with piezoelectricity, an electrode exciting an elastic wave and provided on the first medium, a second medium made of a different material from the first medium and provided on the first medium to cover the electrode, and a sound absorbing portion provided on the second medium.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates generally to elastic boundary wave devices, and more particularly, to an elastic boundary wave device that can suppress an unnecessary response.

2. Description of the Related Art

There are known surface acoustic wave (hereinafter, referred to as SAW) devices, as one of the devices that utilize elastic waves. The SAW devices are for use in various circuits such as, for example, transmit bandpass filters, receive bandpass filters, local filters, antenna duplexers, intermediate frequency (IF) filters, and frequency modulation (FM) modulators, in which wireless signals ranging from 45 MHz to 2 GHz in the frequency band are processed. Those wireless signals are typically used by, for example, mobile telephones.

In recent years, there have been developed elastic boundary wave devices that utilized elastic boundary waves. In an elastic boundary wave device, elastic waves are concentrated in the boundary of two different mediums. Therefore, even if a foreign material is adhered to outer surfaces of the two mediums, there will be no change or degradation in the characteristics, such as the variation of the frequency or the increase in the electric loss, unlike the SAW device.

In the SAW device, there is a problem of an unnecessary response that is never used by a resonator. Therefore, as disclosed in Japanese Patent Application Publication No. 6-112764 (hereinafter, referred to as Document 1), there is provided an absorber or grating at an edge of the substrate in the SAW device to suppress the unnecessary surface waves that have been reflected. Also, as disclosed in Japanese Patent Application Publication No. 4-239210 (hereinafter, referred to as Document 2), the reflection of bulk waves is suppressed by providing a sound absorbing material at an end surface of the substrate of the SAW device. In addition, as disclosed in Japanese Patent Application Publication No. 4-82315 (hereinafter, referred to as Document 3), there is a technique for suppressing the reflection of the bulk wave by providing a step at an end surface of the substrate of the SAW device.

SUMMARY OF THE INVENTION

The present invention has been made in view of the above circumstances and provides an elastic boundary wave device in where an unnecessary response can be suppressed, whereas the unnecessary response is an excited wave other than a boundary wave present in the boundary of different two mediums.

According to one aspect of the present invention, there is provided an elastic boundary wave device including: a first medium with piezoelectricity; an electrode exciting an elastic wave and provided on the first medium; a second medium made of a different material from the first medium and provided on the first medium to cover the electrode; and a sound absorbing portion provided on the second medium.

According to another aspect of the present invention, there is provided an elastic boundary wave device including: a first medium with piezoelectricity; an electrode exciting an elastic wave and provided on the first medium; a second medium made of a different material from the first medium and provided on the first medium to cover the electrode; and a protrusion provided on the second medium.

The sound absorbing portion reduces the energy of the surface wave on the surface of the second or third medium, thereby reducing the unnecessary response caused by the surface wave.

According to yet another aspect of the present invention, there is provided an elastic boundary wave device including: a first medium with piezoelectricity; an electrode exciting an elastic wave and provided on the first medium; a second medium made of a different material from the first medium and provided on the first medium to cover the electrode; and a third medium made of a different material from the first medium and the second medium and provided on the second medium, and the third medium has a thickness of 0.25 times or more of a periodicity of the electrode.

BRIEF DESCRIPTION OF THE DRAWINGS

Exemplary embodiments of the present invention will be described in detail with reference to the following drawings, wherein:

FIG. 1 shows usual insertion loss of the elastic boundary wave device, with respect to frequency;

FIG. 2 is a cross-sectional view of an elastic boundary wave device employed in a first exemplary embodiment of the present invention;

FIG. 3 shows insertion loss of the elastic boundary wave device employed in the first exemplary embodiment of the present invention, with respect to frequency;

FIG. 4 is a cross-sectional view of an elastic boundary wave device employed in a second exemplary embodiment of the present invention;

FIG. 5A through FIG. 5D are first cross-sectional views of an elastic boundary wave device employed in a third exemplary embodiment of the present invention;

FIG. 6A through FIG. 6D are second cross-sectional views of the elastic boundary wave device employed in the third exemplary embodiment of the present invention;

FIG. 7A through FIG. 7D are third cross-sectional views of the elastic boundary wave device employed in the third exemplary embodiment of the present invention;

FIG. 8A and FIG. 8B are cross-sectional views of an elastic boundary wave device employed in a fourth exemplary embodiment of the present invention;

FIG. 9A and FIG. 9B are cross-sectional views of an elastic boundary wave device employed in a fifth exemplary embodiment of the present invention;

FIG. 10A and FIG. 10B are cross-sectional views of an elastic boundary wave device employed in a sixth exemplary embodiment of the present invention;

FIG. 11 is a cross-sectional view of an elastic boundary wave device employed in a seventh exemplary embodiment of the present invention; and

FIG. 12 shows the insertion loss of the elastic boundary wave device employed in the seventh exemplary embodiment of the present invention, with respect to frequency of a surface wave.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1 is a view showing passband characteristics of a one-port resonator having an elastic boundary wave device therein. In addition to a response of the boundary wave at a lower frequency side, an unnecessary response is present at a higher frequency side. When such unnecessary response is present, excellent suppression characteristics are not available in an elastic boundary wave device for use in a filter. The unnecessary response hinders the excellent frequency characteristics. According to the experiment carried out by the inventors of the present invention, such unnecessary response of the elastic boundary wave device is substantially different in the characteristic from the spurious of small cycles, which are found in the conventional SAW devices. For this reason, the unnecessary response cannot be suppressed by providing the sound absorbing material or step at the end surface of the substrate as described in Document 1 through Document 3. As a cause of the unnecessary response, it is believed that the unnecessary response comes from the surface wave of the elastic boundary wave device, and must be a sezawa wave, which is a higher mode of Rayleigh wave, judging from the size of the response. Accordingly, a description will now be given, with reference to the accompanying drawings, of exemplary embodiments of the present invention, with regards to the elastic boundary wave device where the unnecessary response can be suppressed.

First Exemplary Embodiment

A first exemplary embodiment of the present invention is an example in which a sound absorbing portion is provided at a surface. FIG. 2 is a cross-sectional view of an elastic boundary wave device employed in the first exemplary embodiment of the present invention. There is provided an electrode 12 made of a metal such as Cu (copper) or the like on a piezoelectric substrate 10 (first medium) of a rotation-Y plate LN (LiNbO₃) substrate. The electrode 12 is interdigital transducers (IDTs) or comb-like electrodes to excite elastic waves. There is provided on the piezoelectric substrate 10, a silicon oxide film 14 (second medium) made up of a different material from the piezoelectric substrate 10, to cover the electrode 12. There is also provided on the silicon oxide film 14, a sound absorbing portion 20 made of a silicon resin. The silicon oxide film 14 has an acoustic velocity slower than that of the piezoelectric substrate 10. Accordingly, the elastic boundary waves concentrate on and propagate along the boundary between the silicon oxide film 14 and the piezoelectric substrate 10.

FIG. 3 shows insertion loss of the elastic boundary wave device (conventional example) that does not include the sound absorbing portion 20 and that of the elastic boundary wave device employed in the first exemplary embodiment, with respect to frequency. In the elastic boundary wave device employed in the first exemplary embodiment, the unnecessary response due to the surface wave at the higher frequency side is smaller. In accordance with the first exemplary embodiment of the present invention, the sound absorbing portion 20 provided on the silicon oxide film 14 reduces the energy of the surface wave on the surface of the silicon oxide film 14, thereby enabling the unnecessary response resulted from the surface wave to be decreased. Here, the sound absorbing portion 20 is made of a material, for example, resin, which reduces the energy of the surface wave on the surface of the silicon oxide film 14. As a resin, epoxy resin, silicon resin, phenol resin, ultraviolet curing resin, and polyimide resin may be used. In consideration of the mounting process onto the package, the resin preferably has the heat resistance of 150° C. or more. Also, the sound absorbing portion 20 on the silicon oxide film 14 is preferably provided over the whole surface of the silicon oxide film 14. However, the sound absorbing portion 20 may be partially provided on the silicon oxide film 14.

Second Exemplary Embodiment

A second exemplary embodiment of the present invention is an example in which an aluminum oxide film 16 is interposed between the silicon oxide film 14 and the sound absorbing portion 20. Referring now to FIG. 4, the elastic boundary wave device employed in the second exemplary embodiment includes the aluminum oxide film 16 (third medium) is provided between the silicon oxide film 14 and the sound absorbing portion 20. The aluminum oxide film 16 is made of a different material from the piezoelectric substrate 10 and the silicon oxide film 14. In the second exemplary embodiment, the same components and configurations as those employed in the first exemplary embodiment have the same reference numerals and a detailed explanation will be omitted. In accordance with the second exemplary embodiment, there is provided on the silicon oxide film 14, the aluminum oxide film 16 of a higher acoustic velocity than that of the silicon oxide film 14, thereby confining the elastic boundary wave inside the silicon oxide film 14. Also, the sound absorbing portion 20 provided on the aluminum oxide film 16 reduces the energy of the surface wave propagating on the surface of the aluminum oxide film 16, thereby decreasing the unnecessary response caused by the surface wave.

Third Exemplary Embodiment

A third exemplary embodiment of the present invention is an example in which there are provided protrusions on a silicon oxide film or on an aluminum oxide film. Referring now to FIG. 5A, there is provided the electrode 12 on the piezoelectric substrate 10 (first medium). There is provided on the piezoelectric substrate 10, the silicon oxide film 14 (second medium) made of a different material from the piezoelectric substrate 10 so as to cover the electrode 12. There are provided on the silicon oxide film 14, multiple protrusions 22 a made of silicon oxide on a periodic basis. Referring to FIG. 5B, there are provided on the silicon oxide film 14, the aluminum oxide film 16 shown in FIG. 5A, and there are also provided on the aluminum oxide film 16, multiple protrusions 22 b made of aluminum oxide on a periodic basis. Referring to FIG. 5C, there are provided on the silicon oxide film 14, multiple protrusions 22 c on an irregular basis. Referring to FIG. 5D, there are provided on the silicon oxide film 14, the protrusions 22 b made of aluminum oxide on a periodic basis.

Referring to FIG. 6A, there are provided on the silicon oxide film 14, the multiple protrusions 22 a made of silicon oxide, and the multiple protrusions 22 b made of aluminum oxide on a periodic basis. Referring to FIG. 6B, there are provided on the silicon oxide film 14, multiple protrusions 22 d made of aluminum oxide on an irregular basis. Referring to FIG. 6C, there are provided on the silicon oxide film 14, the multiple protrusions 22 c made of silicon oxide and the multiple protrusions 22 d made of aluminum oxide on an irregular basis. Referring to FIG. 6D, there are provided on the aluminum oxide film 16, the multiple protrusions 22 d made of aluminum oxide on an irregular basis, the aluminum oxide film 16 being provided on the silicon oxide film 14.

Referring to FIG. 7A, there are provided on the aluminum oxide film 16, the protrusions 22 a made of silicon oxide, the aluminum oxide film 16 being provided on the silicon oxide film 14 on a periodic basis. Referring to FIG. 7B, there are provided on the aluminum oxide film 16, the multiple protrusions 22 a made of silicon oxide and the multiple protrusions 22 b made of aluminum oxide on a periodic basis, the aluminum oxide film 16 being provided on the silicon oxide film 14. Referring to FIG. 7C, there are provided on the aluminum oxide film 16, the multiple protrusions 22 c made of silicon oxide on an irregular basis, the aluminum oxide film 16 being provided on the silicon oxide film 14. Referring to FIG. 7D, there are provided on the aluminum oxide film 16, the multiple protrusions 22 c made of silicon oxide and the multiple protrusions 22 d made of aluminum oxide on an irregular basis, the aluminum oxide film 16 being provided on the silicon oxide film 14.

In accordance with the third exemplary embodiment of the present invention, there are provided on the silicon oxide film 14, the multiple protrusions 22 a, the protrusions 22 b, the protrusions 22 c, or the protrusions 22 d. The protrusions 22 a, the protrusions 22 b, the protrusions 22 c, or the protrusions 22 d scatter the surface waves propagating on the surface of the silicon oxide film 14 or the aluminum oxide film 16. This enables the unnecessary response caused by the surface wave to be decreased. In the third exemplary embodiment of the present invention, it is configured such that the protrusions 22 a, the protrusions 22 b, the protrusions 22 c, or the protrusions 22 d have heights ranging from 0.5 μm to 2.0 μm. However, the present invention is not limited to this. Also, in the third exemplary embodiment of the present invention, the multiple protrusions 22 a, the multiple protrusions 22 b, the multiple protrusions 22 c, or the multiple protrusions 22 d are provided on the whole surface of the silicon oxide film 14 or on the aluminum oxide film 16. However, it is only necessary to provide at least one of the protrusions 22 a, the protrusions 22 b, the protrusions 22 c, and the protrusions 22 d, and it is only necessary to provide the protrusions partially on the surface of the silicon oxide film 14 or on the aluminum oxide film 16. In addition, the protrusions 22 a, the protrusions 22 b, the protrusions 22 c, and the protrusions 22 d have periodicities, widths, and gaps shown smaller than those of the electrode 12, in FIG. 5A through FIG. 5D, in FIG. 6A through FIG. 6D, and in FIG. 7A through FIG. 7D. However, since six fingers of the electrode 12 are schematically shown, the protrusions 22 a, the protrusions 22 b, the protrusions 22 c, and the protrusions 22 d generally have periodicities, widths, and gaps greater than those of the electrode 12.

As shown in FIG. 5A, FIG. 5 b, FIG. 5D, FIG. 6A, FIG. 7A, and FIG. 7B, the multiple protrusions 22 a or 22 b may have different periodicities from that of the electrode 12. The multiple protrusions 22 a or 22 b have different periodicities from that of the electrode 12, thereby enabling the surface wave leading to an unnecessary response to be scattered. As shown in FIG. 5C, FIG. 6B, FIG. 6C, FIG. 6D, FIG. 7C, and FIG. 7D, the multiple protrusions 22 a or 22 b may be provided on an irregular basis.

As shown in FIG. 5A, FIG. 5C, FIG. 7A, and FIG. 7C, a same material as that of the silicon oxide film 14 may be employed for the protrusions 22 a or the. protrusions 22 c. As shown in FIG. 5B, FIG. 5D, FIG. 6B, and FIG. 6D, a different material from that of the silicon oxide film 14 may be employed for the protrusions 22 b or the protrusions 22 d. As shown in FIG. 6A, FIG. 6C, FIG. 7B, and FIG. 7D, a same material as that of the silicon oxide film 14 may be employed for a portion of the protrusions 22 a or the protrusions 22 c, and a different material from that of the silicon oxide film 14 may be employed for a portion of the protrusions 22 b or the protrusions 22 d.

As shown in FIG. 5B, FIG. 6D, and FIG. 7A through FIG. 7D, the aluminum oxide film 16 may be provided between the silicon oxide film 14 and the protrusions 22 a, the protrusions 22 b, the protrusions 22 c, or the protrusions 22 d. As shown in FIG. 5B and FIG. 6D, a same material as that of the aluminum oxide film 16 may be employed for the protrusions 22 b and the protrusions 22 d. As shown in FIG. 7B and FIG. 7D, a portion of the multiple protrusions may be composed of the protrusions 22 a or the protrusions 22 c, made of a same material as that of the silicon oxide film 14, and another portion thereof may be made of a same material as that of the aluminum oxide film 16. In this manner, the material used for the protrusions may be selected arbitrarily.

Fourth Exemplary Embodiment

A fourth exemplary embodiment of the present invention is an example in which the protrusions include three different materials. Referring now to FIG. 8A, there are provided on the silicon oxide film 14, the multiple protrusions 22 c made of silicon oxide, the multiple protrusions 22 d made of aluminum oxide, and protrusions 22 e made of a different material from silicon oxide or from aluminum oxide on an irregular basis.

FIG. 8B shows another example of the fourth exemplary embodiment of the present invention. There is provided the aluminum oxide film 16 (third medium) between the silicon oxide film 14 and the protrusions 22 c and the protrusions 22 d. In accordance with the fourth exemplary embodiment, three or more different materials may be used for each of the protrusions 22 a through 22 e. In the fourth exemplary embodiment of the present invention, the protrusions 22 c, the protrusions 22 d, and the protrusions 22 d are provided on an irregular basis. However, the protrusions 22 c, the protrusions 22 d, and protrusions 22 e may be provided on a periodic basis.

Fifth Exemplary Embodiment

A fifth exemplary embodiment of the present invention is an example in which the protrusions 22 c and the protrusions 22 d have a plane surface. Referring now to FIG. 9A, there is provided the electrode 12 on the piezoelectric substrate 10 (first medium). There is provided on the piezoelectric substrate 10, the silicon oxide film 14 (second medium) made of a different material from the piezoelectric substrate 10 so as to cover the electrode 12. There are provided on the silicon oxide film 14, the multiple protrusions 22 d made of aluminum oxide on an irregular basis. There are provided the protrusions 22 c made of silicon oxide between the protrusions 22 d.

FIG. 9B shows another example of the fifth exemplary embodiment of the present invention. There is provided the aluminum oxide film 16 (third medium) between the silicon oxide film 14 and the protrusions 22 c and the protrusions 22 d. In accordance with the fifth exemplary embodiment of the present invention, there are provided the protrusions 22 c and the protrusions 22 d made of different materials from each other in contact with each other such that the protrusions 22 c and the protrusions 22 d have a plane surface as a whole. In the fifth exemplary embodiment of the present invention, the protrusions 22 c and the protrusions 22 d are provided on an irregular basis. However, the protrusions 22 c and the protrusions 22 d may be provided on a periodic basis. Also, another material other than the second medium or the third medium may be used for the protrusions 22 c and the protrusions 22 d.

Sixth Exemplary Embodiment

A sixth exemplary embodiment of the present invention is an example in which one or more protrusions and a sound absorbing portion are provided on the silicon oxide film or on the aluminum oxide film. Referring now to FIG. 10A, there is provided the electrode 12 on the piezoelectric substrate 10 (first medium). There is provided on the piezoelectric substrate 10, the silicon oxide film 14 (second medium) made of a different material from the piezoelectric substrate 10 to cover the electrode 12. There are provided on the silicon oxide film 14, the multiple protrusions 22 a made of silicon oxide on a periodic basis. There is also provided on the silicon oxide film 14 and on the protrusions 22 a, the sound absorbing portion 20 made of, for example, silicon resin.

FIG. 10B shows another example of a sixth exemplary embodiment of the present invention. There is provided the aluminum oxide film 16 (third medium) between the silicon oxide film 14 and the protrusions 22 a. In accordance with the sixth exemplary embodiment, the sound absorbing portion 20 decreases the energy of the surface waves propagating on the surface of the silicon oxide film 14 and on the surface of the protrusions 22 a, or on the surface of the aluminum oxide film 16 and on the surface of the protrusions 22 a.

Seventh Exemplary Embodiment

A seventh exemplary embodiment of the present invention is an example in which the aluminum oxide film 16 is thicker. Referring now to FIG. 11, an elastic boundary wave device employed in the seventh exemplary embodiment includes the electrode 12 arranged on the piezoelectric substrate 10 (first medium). There is provided on the piezoelectric substrate 10, the silicon oxide film 14 (second medium) made of a different material from that of the piezoelectric substrate 10 to cover the electrode 12. There is provided on the silicon oxide film 14, the aluminum oxide film 16 (third medium) to be thicker. FIG. 12 is an enlarged view of a surface wave response in the passband characteristics of the one-port resonator having the elastic boundary wave device therein when a thickness h of the aluminum oxide film 16 is changed. When the thickness h of the aluminum oxide film 16 is gradually made thicker 0.25λ, 0.3λ, and 0.35λ where the electrode 12 has a periodicity of λ, the response from the surface wave is decreased. Also, when the thickness h of the aluminum oxide film 16 is smaller than 0.25λ, the boundary wave is attenuated. This is not desirable. As stated, it is preferable that the thickness h of the aluminum oxide film 16 be thick. In particular, an unnecessary response can be suppressed by causing the thickness h of the aluminum oxide film 16 to be 0.25 times or more of the periodicity λ. In addition, in the elastic boundary wave device used in FIG. 9A and FIG. 9B, the silicon oxide film 14 has a thickness of 500 nm, and the electrode 12 has a periodicity of 2,000 nm.

In the first through seventh exemplary embodiments of the present invention, a description has been given as an example where the LN piezoelectric substrate 10 serves as the first medium, the silicon oxide film 14 serves as the second medium, the aluminum oxide film 16 serves as the third medium, and Cu is used for the electrode 12. Optionally, the first medium may be made of a material with piezoelectricity such as a LiTaO₃ substrate or the like, and the second medium may be made of a different material from the first medium. Preferably, the second medium is made of a material having a slower sound velocity than those of the first and third mediums, since the elastic boundary wave is confined in the periphery of the boundary. Also, it is only necessary for the electrode 12 made of a conductive material. Preferably, a metal of high density such as Cu or Au is employed for the electrode 12 so as to suppress the reflection of the elastic boundary wave.

Although a few specific exemplary embodiments employed in the present invention have been shown and described, it would be appreciated by those skilled in the art that changes may be made in these exemplary embodiments without departing from the principles and spirit of the invention, the scope of which is defined in the claims and their equivalents.

The present invention is based on Japanese Patent Application No. 2006-061693 filed on Mar. 7, 2006, the entire disclosure of which is hereby incorporated by reference. 

1. An elastic boundary wave device comprising: a first medium with piezoelectricity; an electrode exciting an elastic wave and provided on the first medium; a second medium made of a different material from the first medium and provided on the first medium to cover the electrode; and a sound absorbing portion provided on the second medium.
 2. The elastic boundary wave device as claimed in claim 1, further comprising a third medium provided between the second medium and the sound absorbing portion, wherein the first medium and the second medium are made of different materials.
 3. An elastic boundary wave device comprising: a first medium with piezoelectricity; an electrode exciting an elastic wave and provided on the first medium; a second medium made of a different material from the first medium and provided on the first medium to cover the electrode; and a protrusion provided on the second medium.
 4. The elastic boundary wave device as claimed in claim 3, wherein there are provided multiple protrusions having a different periodicity from the electrode.
 5. The elastic boundary wave device as claimed in claim 3, wherein there are provided multiple protrusions on an irregular basis.
 6. The elastic boundary wave device as claimed in claim 3, wherein the protrusion is made of a same material as the second medium.
 7. The elastic boundary wave device as claimed in claim 3, wherein the protrusion is made of a different material from the second medium.
 8. The elastic boundary wave device as claimed in claim 3, wherein the there are provided multiple protrusions, and a portion of the multiple protrusions is made of a same material as the second medium and another portion of the multiple protrusions is made of a different material from the second medium.
 9. The elastic boundary wave device as claimed in claim 3, further comprising a third medium provided between the second medium and the protrusion, wherein the first medium and the second medium are made of different materials.
 10. The elastic boundary wave device as claimed in claim 9, wherein the protrusion is made of a same material as the third medium.
 11. The elastic boundary wave device as claimed in claim 9, wherein there are provided multiple protrusions, and a portion of the multiple protrusions is made of a same material as the second medium and another portion of the multiple protrusions is made of a same material as the third medium.
 12. An elastic boundary wave device comprising: a first medium with piezoelectricity; an electrode exciting an elastic wave and provided on the first medium; a second medium made of a different material from the first medium and provided on the first medium to cover the electrode; and a third medium made of a different material from the first medium and the second medium and provided on the second medium, wherein the third medium has a thickness of 0.25 times or more of a periodicity of the electrode. 